Physics Department News

The colour of a chemical compound is just about top of the list of its most-easily measured properties, a basic measurement requiring no specialised equipment beyond one's eyes, and perhaps a colour chart for comparison. To put this on a more quantitative footing, one can carefully measure the experimental absorption spectrum of a compound, convolve it with the known response functions of each of the three colour receptors in the human eye (red, green and blue), and exactly reproduce a sample of this colour.

However, despite many decades of continual advancement in our ability to predict the properties of materials from first principles, using quantum mechanical simulation, it still remains extremely challenging to predict colours, and more generally optical absorption spectra, particularly in a complex environment such as a liquid solvent. This capability would be of great value in numerous fields, including the food manufacturing, pharmaceutical and textile industries. It would allow rapid identification of potential impurity molecules, and rapid screening of candidate molecules based on their colour, without needing to synthesise them.

One of the most challenging aspects of this kind of theoretical spectroscopy is understanding how the excitations of a molecule shift when it is placed in the environment of a solvent. The excitation will “hybridise” with excitations of the surrounding environment, and spread into it to some extent, resulting in a so-called “solvatochromic shift”, namely a shift of absorption energies compared to their value in vacuum. However, the calculations required to capture this effect need to be performed on a large, realistic cluster of molecule plus solvent, requiring a model that is very large by the usual standards quantum-mechanical modelling (thousands of atoms).

Using time-dependent DFT calculations on an unprecedented scale, a team of researchers from Cambridge, Imperial and Warwick, led by Dr Nicholas Hine, has used the ONETEP Linear-Scaling DFT code (www.onetep.org) to investigate how the solvatochromic shift of a widely-used red dye (Alizarin) behaves in aqueous solvent. This is a vital step towards accurate colour prediction from first principles.

This work has recently been published in the Journal of Chemical Theory and Computation: Solvent effects on electronic excitations of an organic chromophore, T. J. Zuehlsdorff, P. D. Haynes, F. Hanke, M. C. Payne, and N. D. M. Hine J. Chem. Theory Comput., Accepted (2016).

http://pubs.acs.org/doi/abs/10.1021/acs.jctc.5b01014

• Image caption: A model of alizarin, its excitations in various models of solvent, and the slow convergence with respect to size of the region of explicit solvent modelled.